Controlling the local-ensemble structure in mesoporous hybrid titania-silica thin films containing aminopropyl groups

Abstract

Mesoporous hybrid materials containing inorganic and organic functional groups are relevant for advanced applications in separation, sorption or heterogeneous catalysis. The possibility of combining materials with high surface area and tailorable mesopore size with a mixed oxide framework and organic functions open the gate to imitating complex biosystems such as enzyme active sites. One of the critical aspects towards a multiscale control of these complex materials is the understanding of the actual framework structure and the interplay of the framework ions and the organic functions, and how these features are related to the sol-gel preparation conditions. In this work, mesoporous hybrid organic-inorganic thin films (MHTF) based on a mixed silica-titania matrix containing 20% aminopropyl functions were prepared and thoroughly studied by X-ray absorption spectroscopy at both the Ti and Si K-edges, and by O1s and N1s X-ray photoelectron spectroscopy. This approach permitted us to simultaneously probe the changes in Si and Ti coordination, the linkages between the inorganic centers, and the availability of the amino functions along samples with varying Si:Ti ratio. We find substantial differences between the local structure of pure inorganic oxides and the hybrid materials. In the oxides, increasing the %Ti leads to an increase in octahedral Si sites and Ti-oxo clusters with shorter Ti-O bonds. In the hybrid materials, higher Ti coordination with longer bonds are observed, along with a prevalence of Si centers with distorted tetrahedral coordination. These findings suggest that aminopropyl building blocks act as a compatibilizer between Ti(IV) and Si(IV) centers, leading to a hybrid mixed phase with large silica-titania interface. This intermixing also influences the exposition of amino groups at the pore surface. These aspects are of importance in the design of high surface area adsorbents, permselective membranes or heterogeneous catalysts